130 ELEMENTS OF APPLIED MICROSCOPY. 
serum test for typhoid fever. If human blood-serum be 
injected into the lymph spaces of a rabbit, there appears 
in the blood of the rabbit a substance which precipitates 
certain albuminoids in the blood of man; and this reac- 
tion serves to distinguish human blood from that of the 
domestic animals with certainty. Only with the cor- 
puscles of apes and monkeys does the serum of the immu- 
nized rabbit react in a similar manner. This test, which 
ordinarily bears the name of Bordet, who first suggested 
it, is not applied under the microscope, but in small tubes 
in which the precipitation is detected macroscopically. 
3. The Microspectroscope and Its Use.—In testing 
blood-stains, and in many other fields of microbiology 
and microchemistry, the spectroscope may furnish aid. 
As ‘ordinarily used, the instrument consists of a tube 
fitting on the eyepiece of a microscope and containing, 
as shown in Fig. 49, a triangular prism of flint glass 
which disperses: white light into its constituent colors 
and two prisms of crown glass set in the opposite direc- 
tion, which serve to counteract the refraction produced 
by the first prism. The principle is the same as that 
of the achromatic objective, but whereas in the latter 
the angles are arranged to produce a net refraction with 
neutralized dispersion, the prisms of the spectroscope 
yield a net dispersion with practically no refraction. 
The special type of eyepiece used with the micro- 
spectroscope contains a diaphragm which only allows a 
longitudinal slit of light to pass, and the width of the 
slit is regulated by a set screw, being cut down for use 
to half a millimeter or less. When the detachable prism 
